1. Field of the Invention
The invention relates to a method for coating powder material by means of an adhesive for generative rapid prototyping processes, in particular 3D binder printing, and also the production of bodies from a coated powder material by means of generative rapid prototyping processes and its preferred use in foundry practice and precision engineering.
2. Related Art of the Invention
Among the more recent and particularly interesting powder-based generative rapid prototyping (RP) processes is the 3D binder printing process.
In a first variant of 3D binder printing, a layer of particles or granules is discharged onto an underlying surface and moistened with a binding liquid in predetermined regions, which respectively correspond to a layer of an object to be generated. The particles are wetted and adhesively bonded by the binder liquid in the moistened regions. Subsequent evaporation of the solvents in the binder liquid has the effect that the particles adhere directly to one another, fusing together at least in their edge regions. 3D binder printing processes that are related in particular to this type are known for example from European patents EP 0 644 809 B1, EP 0 686 067 B1 and European patent application EP 1 099 534 A2.
In a further variant of this method, use is made of a binder liquid containing sintering aids which, when the moistened regions are dried, leaves behind sintering aids which make it possible for the particles to be sintered as a solid mass in a subsequent sintering treatment. Hard and compact sintered bodies can be obtained in this way.
EP 0 925 169 B1 discloses a further variant of the 3D binder printing process in which mixtures of particles, fillers and adhesives are used. The binder liquid is substantially formed just by a solvent for the adhesive contained in the mixture. The adhesive may in this case also be in the form of a coating of the particles. In this respect, water-soluble polymers as powder coating and aqueous binder liquids are known. EP 1163999 A2 discloses coated particles, EP 0897745 A1 discloses coating methods.
A further process of generative rapid prototyping (RP) provides for the particles to be made to solidify in the layer by means of light, or laser-induced sintering, for example according to WO 03 106146 A1 (hereafter laser sintering). For this purpose, the layer is heated in defined regions by means of exposure to light, if appropriate also through a mask, or laser radiation to the extent that the particles can fuse together or sinter together.
The known binder printing processes have the disadvantage that the finished body exhibits a distinct shrinkage in comparison with the region originally moistened with the binder liquid. In binder printing, the particles move closer together when they are moistened, under the effect of the capillary forces and surface tension, in particular whenever adhesives that may be present are dissolved by the binder liquid, so that a distinct shrinkage has taken place already during the shaping or after the drying to form the green compact. A shrinkage in the cured regions also takes place in the case of laser-induced sintering.
The known adhesives or adhesive coatings have the disadvantage that they are hydrophilic and therefore absorb water from the surroundings, in particular from the atmospheric moisture. This generally leads to an undesired agglomeration of the particles. In the case of 3D binder printing, there is also a great absorption of binder liquid during the printing. This has a negative influence on the distinctness of image and the storage stability, and also the handling of the powder. Agglomerates lead to uneven layers of powder and defects in the bodies formed from them. For the production of homogeneous bodies of a constant quality, the pourability of the powders is also of great significance.